In recent years, small-sized electronic devices represented by mobile terminals and the like have been widely spread and further down-sizing, lighter weight and longer life are strongly demanded. To a market demand like this, secondary batteries capable of obtaining, in particular, a smaller size, a lighter weight and a higher energy density have been developed. The secondary batteries are studied to apply also to, without limiting to small-sized electronic devices, large-sized electronic devices represented by automobiles and power-storage systems represented by houses or the like.
Among these, a lithium ion secondary battery is highly expected because it may readily obtain a smaller size and higher capacity and may obtain the energy density higher than that of a lead battery or a nickel-cadmium battery.
The lithium ion secondary battery includes an electrolytic solution together with a positive electrode and a negative electrode, and a separator, and the negative electrode includes a negative electrode active material involving a charge/discharge reaction.
As the negative electrode active material, while a carbon material is widely used, a further improvement in the battery capacity is demanded from recent market demand. In order to improve the battery capacity, it has been studied to use silicon as the negative electrode active material. This is because a great improvement of the battery capacity may be expected since silicon has a theoretical capacity (4199 mAh/g) no smaller than 10 times the theoretical capacity (372 mAh/g) of graphite. A development of a silicon material as the negative electrode active material includes studies on not only a silicon simple substance but also compounds represented by alloys, oxides or the like. Further, shapes of the active material have been studied, from a coating type, which is standard for the carbon material, to an integrated type directly deposited on a current collector.
However, when, as the negative electrode active material, the silicon is used as a main raw material, since the negative electrode active material expands and contracts during charging/discharging, mainly in the neighborhood of a superficial layer of the negative electrode active material, crack tends to occur. Further, an ionic substance is generated inside of the active material, the negative electrode active material tends to crack. When a superficial layer of the negative electrode active material is broken, a new surface is generated thereby, and a reaction area of the active material increases. At this time, since a decomposition reaction of an electrolytic solution occurs on the new surface and a coating that is decomposition product of the electrolytic solution is formed on the new surface, the electrolytic solution is consumed. Therefore, cycle characteristics tends to be degraded.
Until now, in order to improve a battery initial efficiency and the cycle characteristics, negative electrode materials for lithium ion secondary batteries having the silicon material as a main material and electrode configurations have been variously studied.
Specifically, in order to obtain excellent cycle characteristics and high safety, silicon and amorphous silicon dioxide are simultaneously deposited by using a gas phase method (see, for example, Patent Document 1). Further, in order to obtain high battery capacity and safety, a carbon material (an electron conductor) is provided on a superficial layer of particles of silicon oxide (see, for example, Patent Document 2). Further, in order to improve the cycle characteristics and to obtain high input/output characteristics, an active material containing silicon and oxygen is prepared and an active material layer having a high oxygen ratio in the neighborhood of a current collector is formed (see, for example, Patent Document 3). Still further, in order to improve the cycle characteristics, oxygen is contained in a silicon active material such that an average oxygen content is not higher than 40 at. %, and an oxygen content is high in a position close to a current collector (see, for example, Patent Document 4).
Further, in order to improve an initial charge/discharge efficiency, a nano composite containing a Si phase, SiO2 and a MyO metal oxide is used (see, for example, Patent Document 5). Still further, in order to improve the cycle characteristics, SiOx (0.8≤x≤1.5, particle size range=1 μm to 50 μm) and a carbon material are mixed and sintered at a high temperature (see, for example, Patent Document 6). Further, in order to improve the cycle characteristics, a mol ratio of oxygen to silicon in a negative active material is set to 0.1 to 1.2, and an active material is controlled in the range such that a difference of a maximum value and a minimum value of the mol ratio in the neighborhood of an interface of the active material and a current collector is not larger than 0.4 (see, for example, Patent Document 7). Still further, in order to improve battery load characteristics, a metal oxide containing lithium is used (see, for example, Patent Document 8). Further, in order to improve the cycle characteristics, a hydrophobic layer such as a silane compound is formed on a superficial layer of a silicon material (see, for example, Patent Document 9).
Still further, in order to improve the cycle characteristics, a silicon oxide is used, and a graphite coating is formed on a superficial layer thereof to impart conductivity (see, for example, Patent Document 10). In the Patent Document 10, regarding a shift value obtained from a Raman spectrum of the graphite coating, broad peaks appear at 1330 cm−1 and 1580 cm−1, and an intensity ratio thereof I1330/I1580 is 1.5<I1330/I1580<3. Further, in order to improve high battery capacity and cycle characteristics, particles having a silicon crystallite phase dispersed in a silicon dioxide are used (see, for example, Patent Document 11). Still further, in order to improve overcharge and overdischarge characteristics, a silicon oxide in which atomic ratios of silicon and oxygen are controlled to 1:y (0<y<2) is used (see, for example, Patent Document 12). Further, in order to improve high battery capacity and cycle characteristics, a mixed electrode of silicon and carbon is prepared and a silicon ratio is designed to be not lower than 5 wt % and not higher than 13 wt % (see, for example, Patent Document 13).